This invention relates to a combined valve suitable for use in a reheating steam turbine which combines a stop valve with a control valve.
In one type of steam turbine plant, a turbine bypass system is provided to reduce the time required for starting the steam turbine. Startup of a steam turbine is effected by first introducing the reheating steam generated in the reheater of a boiler to a medium pressure turbine through a combined reheating valve which combines a reheating stop valve with an intercepting valve in the same valve casing, to thereby raise the speed of operation of the turbine and apply an initial load. In the turbine startup operation of this system, it is essential that the flow rate of reheating steam be precisely controlled by means of the combined reheating valve. The of an intercepting valve of a large diameter use for effecting control of the flow rate of steam in normal operating condition would make it impossible to effect precise control as desired. Therefore, a proposal has been made to use a combined reheating valve, as described in Japanese Utility Model Application Laid-Open No. 4378/80, which combines a reheating stop valve with a bypass valve for effecting control of a flow rate of low order.
This combined reheating valve is constructed such that an intercepting valve has its head loosely fitted through a seal ring to the inner periphery side of a cylindrical projection of a valve casing to define a chamber therebetween, and a reheating stop valve having a built-in bypass valve is arranged on the lower end of the intercepting valve which is located on the downstream side with respect to the stream of steam.
When the steam turbine is started, reheating steam is introduced through a gap between the seal ring and the intercepting valve into the chamber, to fill the upstream side of the reheating stop valve with the reheating steam through a balance hole formed in the intercepting valve communicating with the chamber. By opening the bypass valve of the reheating stop valve while in this condition, the reheating steam is introduced into the medium pressure turbine and the turbine is started.
However, the combined reheating valve described hereinabove has some disadvantages. One of them is that the flow rate of the reheating steam flowing downwardly at turbine startup is limited, and another disadvantage is that it is necessary to switch the valve operation between the reheating valve and the intercepting valve at turbine startup or, more specifically, to first actuate the reheating stop valve to open the reheating stop valve having the built-in bypass valve and then actuate the intercepting valve for opening the intercepting valve.
The disadvantage that is first mentioned is such that since the intercepting valve is brought to a full close condition at initial stages of turbine startup, the volume of the reheating steam flowing through the gap between the seal ring and the intercepting valve is restricted, so that the steam necessary in the process of turbine startup .fwdarw. increase of turbine operation speed .fwdarw. actuation of turbines in combination (application of an initial load) is not positively provided.
The last mentioned disadvantage is such that when the intercepting valve is first fully opened before the turbine is started, the problem that the steam would be lacking in volume as stated with regard to the first mentioned disadvantage can be eliminated, but it becomes necessary to switch the valve operation to the intercepting valve which effects control of the flow rate of reheating steam during normal operation after the process of turbine startup .fwdarw. increase of turbine operation speed .fwdarw. actuation of turbines in combination (application of an initial load) has been completed. Particularly, since the reheating stop valve is constructed such that it cannot be opened until the pressure differential between the front and the rear of the valve is reduced below a predetermined level as the reheating stop valve serves as a sort of safety valve, it is necessary to adjust the opening of the intercepting valve so as to control the pressure in the front of the reheating stop valve or the pressure at the rear of the intercepting valve by detecting the pressure at the rear of the reheating stop valve when switching of valve operation is effected.
Moreover, when switching of valve operation is effected, it is necessary to bring the area of opening of the bypass valve built in the reheating stop valve into complete coincidence with the area of opening of the intercepting valve that is to be closed. Otherwise, the flow rate of the reheating steam would show a variation and it would become impossible to effect control of the turbine. It would be difficult to carry out valve operation that would satisfy the aforesaid requirement.